[Cryptography] Asynchronous forward secrecy encryption

[Michael Rogers]

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Hi Felix,

It seems to me that your suggestion works, given some assumptions:

1. Long-lived process on the receiving client
2. Long-lived process on the server
3. Receiving client can contact server regularly to upload keys
4. Sending client can contact server before sending to receive a key
5. Receiving client can contact sender if message can't be decrypted
(presumably this response isn't encrypted, to avoid infinite regress)
6. Some kind of protection against key exhaustion attacks

Those seem like reasonable assumptions on the internet, but perhaps
not for other settings like mesh networks or DTNs.

Any thoughts about how to achieve number 6?

Cheers,
Michael

On 29/10/13 17:49, Felix Ruzzoli wrote:
> I believe that the memory on current smartphones running android
> is stable enough for the purpose.
> 
> I have a setup where the server stores some pre-generated DH keys
> in memory only. Clients would query for a key for every message
> they want to send in order to complete the DH agreement and send
> the encrypted message plus the public key needed for the other
> party's part of the protocol.
> 
> So obviously there is a small gap. If the process has been killed
> in between, the private key for decrypting that last message is
> lost. The best we can do in that case is to tell the other party
> that we could not decrypt that last message and they'd have to
> resend with a fresh key we just sent along.
> 
> Apart from the problem stated in the last paragraph, that would
> enable PFS with asynchronous messages with a relatively simple
> protocol. Or am I missing something here?
> 
> Regards, felix
> 
> _______________________________________________ The cryptography
> mailing list cryptography at metzdowd.com 
> http://www.metzdowd.com/mailman/listinfo/cryptography

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